This invention relates to a novel class of polypropylenes that possess elastic character and are hybrid polymers having stereregularity intermediate between those that are highly syndiotactic (or highly isotactic) and those that are atactic, as well as catalysts that produce these hybrid polymers.
Elastic polypropylene was first isolated by solvent extraction of polypropylene obtained by Ziegler-Natta polymerization of propylene (inter alia, U.S. Pat. No. 3,175,999). A process for preparing primarily isotactic, fractionable, elastomeric polyolefins has been described (U.S. Pat. No. 4,335,225), using a catalyst prepared by reacting a solid support such as alumina with, e.g., a tetraalkylzirconium compound. U.S. Pat. No. 5,594,080 discloses preparation of a stereoblock elastomeric polypropylene using a metallocene catalyst containing two unbridged phenylindenyl ligands connected to zirconium, i.e. (phenylindenyl)2ZrCl2. Chien et al. (J. Am. Chem. Soc., 112, 2030 (1990); Macromolecules, 25, 1242 (1992); Macromolecules, 24, 850 (1991)) prepared a thermoplastic, elastomeric polypropylene containing about 30% crystalline isotactic polypropylene, using a metallocene catalyst.
Collins et al. (Macromolecules, 28, 3771 (1995); Macromol. Symp., 98, 223 (1995)) describe the use of zirconium and hafnium metallocene catalysts to prepare elastic polypropylene. These polymers had 29-54% mmmm pentads, i.e. they were predominantly isotactic. Significantly, elastic polypropylene could only be obtained when the molecular weight was greater than 50,000 and when the mmmm content of the polymer exceeded 38%, using exclusively hafnium complexes.
European Patent Application No. 666,267 describes the use of a metallocene catalyst of the type {fluorenyl-C2H4-indenyl}ZrCl2 to prepare polypropylene in the presence of hydrogen. Polymer properties are not described. The same catalyst was used (Organometallics, 13, 647 (1994)) to polymerize propylene in toluene in the absence of hydrogen; highly isotactic polymers having mmmm contents ranging from 38 to 64% and melting points of 104-110xc2x0 C. were obtained. When the C2H4 bridging group was replaced by xe2x80x94SiMe2xe2x80x94 (U.S. Pat. No. 5,391,789), only a viscous, tacky oil was obtained.
European Patent Application No. EP 707,016 describes a catalyst system comprising {2-Me-4-naphthyl-indenyl-SiMe2-fluorenyl}ZrCl2, a Lewis acid compound and an organoaluminum compound, to prepare elastomeric polypropylene. U.S. Pat. No. 5,516,848 describes a process for making elastomeric polypropylenes employing a mixture of two different metallocene catalysts, one of which was a monocyclopentadienyl transition metal compound.
Metallocene catalysts generally can be defined as catalysts comprising one or more cyclopentadienyl groups, including substituted cyclopentadienyl groups, in combination with a transition metal, typically a metal of Group IVB of the Periodic Chart (i.e., titanium, zirconium, and hafnium). Useful metallocenes have been described comprising two cyclopentadienyl-type rings, in which both rings are identical (symmetrical metallocenes) and in which the two rings are different, due either to differing substitution patterns on identical ring systems or to the presence of two different ring systems (unsymmetrical metallocenes). The first reports of the use of cyclopentadienyl metallocenes to catalyze the polymerization of 1-olefins appeared in 1957. British Patent Application GB 934,281 first described metallocene catalysts of the type (R-indenyl)2MX2 and (R-fluorenyl)2MX2, where R represents an alkyl or aryl substitutent on the aromatic ring, M represents titanium, zirconium or hafnium, and X represents a halide or an alkyl or alkoxy group, or the like, having from 1 to 12 carbon atoms. Since then, numerous patents and other publications have provided ample evidence of the commercial interest in these materials.
Four limiting cases illustrate the possibilities for stereoregularity in polypropylene. The isotactic structure is typically described as having the methyl side groups oriented so that they are all on the same side of the plane containing the polymer backbone. Another way of describing the isotactic structure employs Bovey""s NMR (nuclear magnetic resonance) nomenclature: An isotactic diad, represented by m (for meso), is a pair of propylene units placed in the polymer chain so that both methyl side groups lie on the same side of the plane containing the polymer backbone. Similarly, a mmmm pentad represents five monomer units with their methyl groups all oriented the same way. 13C NMR can be used to determine the relative amounts of the various pentads present in a propylene polymer.
In contrast, in the syndiotactic structure, the methyl groups are oriented alternately above and below the plane containing the polymer backbone. A pair of propylene units with this up-down arrangement comprises an r (for racemic) diad, and rrrr would correspond to five similarly arranged monomer units.
Hemi-isotactic polypropylene is similar to the isotactic kind except that every other methyl side group has a random orientation.
Finally, in the atactic polymer, the orientation of methyl groups with respect to the plane containing the polymer backbone is random and the number of r and m diads is the same.
The physical properties of polypropylene depend greatly on polymer stereoregularity. For example, isotactic and syndiotactic polypropylene are crystalline polymers that are insoluble in hydrocarbon solvents such as cold xylene. Because of the presence of crystallites that scatter light, they are often opaque or translucent. Additionally, these polymers are stiff, inelastic, and high-melting, their melting points being 171 and 138xc2x0 C., respectively.
In contrast, hemi-isotactic and atactic polymers are non-crystalline and soluble in cold xylene. Atactic polypropylene is transparent, flexible and elastomeric.
Because isotactic and syndiotactic polypropylenes are such useful materials, their preparation has been the object of extensive research. For example, catalysts that produce isotactic polypropylene are disclosed in U.S. Pat. Nos. 4,794,096 and 4,975,403; catalysts that lead to syndiotactic polyolefins are described in U.S. Pat. Nos. 3,258,455, 3,305,538, 3,364,190, 4,892,851, 5,155,080 and 5,225,500.
The precise effect of changes in catalyst structure on catalyst activity and polymer stereoregularity is difficult to predict. For example, U.S. Pat. No. 5,459,218 describes catalysts of the type {flu-bridge-Cp}ZrCl2, wherein flu=fluorenyl, bridge=Me2Si or Ph2Si, and wherein Me=methyl, Ph=phenyl, and Cp=cyclopentadienyl. It is broadly stated and claimed that the propylene polymers contain at least 50% rr content, whereas the syndiotactic content actually achieved ranged from 74 to 82%. The highest molecular weight given for the propylene polymers is 66,000. No unusual properties of these polymers were disclosed nor were any clues provided as to how the catalysts could be modified to produce polymers having a lesser or greater syndiotactic content. By comparison, U.S. Pat. No. 4,892,851 describes a catalyst of a very similar type, namely {flu-CMe2-Cp}ZrCl2, which is reported to produce even more highly syndiotactic (92% rr) polypropylene.
A relationship between catalyst symmetry and polypropylene stereostructure has been proposed (Trends in Polymer Science, 2, 158 (1994)). Symmetry elements, typically described in terms of xe2x80x9cpoint groupsxe2x80x9d of a bis(fluorenyl)MX2-type catalyst are shown in FIGS. 1 and 2 (comparative). FIG. 1 illustrates the different symmetry elements that can occur in metallocene catalysts and FIG. 2 provides illustrative examples. A catalyst (FIG. 1 and, e.g., FIG. 2 entry 20) having point group C2v has three symmetry elements: the structure is unchanged after a 180 degree rotation about the bisector of the MX2 plane and there are two mirror planes of symmetry, reflections in which leave the structure unchanged. One contains the MX2 plane "sgr"h and the other is orthogonal to and bisects the MX2 plane "sgr"v. Looking straight on at the MX2 plane, one sees that the top and bottom as well as the left and right hand side of the catalyst molecule are the same. A catalyst 21 having point group C2 (entry 21 in FIG. 2) contains only a C2 axis: the top and bottom of the molecule are the same, so that 180 degree rotation about the bisector of the MX2 plane leaves the structure unchanged. Catalyst 22 having point group CS contains only a mirror plane of symmetry orthogonal to and bisecting the MX2 plane. Viewed as described above, the left- and right hand sides of the catalyst are the same but the top and bottom are different. Catalyst 25 of C1 point group symmetry contain none of these three symmetry elements.
According to FIG. 2, catalysts of point group C2v are said to produce atactic polypropylene, except that Cp2TiPh2 (24) atypically produces a stereoblock isotactic polymer (perhaps by a different polymerization mechanism) at low temperatures. Metallocenes having C2 and CS point group symmetry are said to produce isotactic and syndiotactic polymers, respectively. However, U.S. Pat. No. 4,769,510 describes catalysts of the type {ind-bridge-ind}MX2, which exists in two isomeric forms. One form is chiral and consists of a d,1 pair of enantiomers; it produces isotactic polypropylene. The second (meso) form is achiral and produces atactic polypropylene (Organometallics, 14, 1256 (1995)). Catalysts with C1 symmetry are said to produce either hemi-isotactic 23 (in FIG. 2) or stereoblock isotactic-atactic 25 (in FIG. 2) polypropylene.
European Patent Application No. 537,130 describes the effect of the size of an organic group attached to a cyclopentadienyl ring in metallocenes of the type {Cp-CMe2-flu}ZrCl2 (Cs point group symmetry), which was shown to produce syndiotactic polypropylene. Introduction of a methyl group into the 3-position of the cyclopentadienyl ring led to {3-MeCp-CMe2-flu}ZrCl2 (C1 point group symmetry), which produced a hemi-isotactic polymer. When the 3-CH3 group was replaced by a t-butyl group to form {3-t-BuCp-CMe2-flu}ZrCl2 (also C1 point group), isotactic polypropylene was obtained instead.
U.S. Pat. No. 5,459,218 relates to syndiotactic polypropylene prepared using silyl bridged metallocenes and having molecular weights up to 66,000. U.S. Pat. No. 5,668,230 discloses certain specific ethylene bridged fluorenyl-containing metallocenes to catalyze olefins polymerization.
One skilled in the art can only conclude that there are no reliable, general correlations between the structure of a metallocene catalyst and the stereospecificity or the stereoregularity of polymers produced thereby.
Briefly, the present invention provides propylene homopolymers that are elastomeric and are soluble in at least one nonpolar organic solvent selected from the group consisting of toluene, xylene, heptane, and hexane, the polymer comprising
a) greater than 3 weight percent and up to 45 weight percent of homotactic sequences, each having only r or m diads, all of which homotactic sequences have a helical length in the range of about 20 to 150 xc3x85, and
b) in the range of 55 to 97 weight percent of the total composition being the sum of
1) homotactic sequences, each having only r or m diads, and being less than 20 xc3x85 in helical length and having fewer than 10 repeat units with mmmm pentads being present in the range of 0 to 35 (preferably 0 to 31) weight percent of the total composition, and
2) heterotactic sequences having unequal numbers of r and m diads,
the polymer having a weight average molecular weight (Mw) of at least about 70,000, preferably greater than 70,000 and up to 2,000,000, more preferably 75,000 to 1,000,000, and most preferably 80,000 to 500,000. The two b) components can be present in any amount so long as their sum amounts to 55 to 97 weight percent of the total composition.
The propylenes of the invention include homotactic sequences of length as specified above having all r or all m diads. As is appreciated by those skilled in the art, these homotactic sequences can assume a helical configuration. Connecting these homotactic sequences are similar homotactic sequences of less than 20 xc3x85 in helical length and having less than 10 repeat units as well as heterotactic sequences having both r and m diads present in unequal numbers. These polymers exhibit elastomeric character that varies from a stiff rubber to that of a very stretchy rubber band. The homotactic sequences can be all r or all m and preferably they are all r, i.e., syndiotactic. In general, the polymers comprise discontinuous small (20 xc3x85 to 150 xc3x85 in length) hard segments in a continuous softer matrix.
The length of the homotactic sequences can be estimated in the following way. Helical chains of polypropylene run approximately parallel to the c-axis of the unit cell of orthorhombic syndiotactic polypropylene. Four monomer units are contained in the unit cell which is 7.6 xc3x85 long. Therefore, each molecule of propylene contributes 7.6/4 or 1.9 xc3x85 to the chain length. This is a reasonable estimate because if the chain were fully extended, each propylene would contribute the distance of two Cxe2x80x94C bonds or 3 xc3x85 to the chain length.
The heterotactic sequences comprise both r and m diads and are of variable lengths and are randomly dispersed as to size and distribution in the polymer chain.
Preferably, the propylene polymers of the invention also have one or more of the following characteristics:
i) a stereoregularity index between 1.30 and 10.0, preferably 1.30 to 7.00 and more preferably 1.60 to 6.40.
ii) a heat of fusion (xcex94Hfus) that is less than 50% of the xcex94Hfus of 100% isotactic polypropylene when mm greater than rr; or less than 50% of the xcex94Hfus of 100% syndiotactic polypropylene when rr greater than mm, (xcex94Hfus values are as given in J. Brandrup et al., POLYMER HANDBOOK, 3d Edition, John Wiley and Sons, N.Y. (1989), section V, p. 29); and
iii) optical clarity.
In another aspect, this invention provides blends of two or more different propylene polymers, at least one of which exhibits the properties described above. Blends of one or more propylene polymers as described above can further comprise crystalline or amorphous polypropylene, tackifying resins, antioxidants, fillers, and other adjuvants known in the art.
In yet another aspect, this invention provides copolymers of propylene wherein the propylene sequences are as defined above, and wherein the comonomers can be olefins having 2 to 20, preferably 2 to 8, carbon atoms. Copolymers can be blended with one another as well as with homopolymers which provides a way to tailor or modify polypropylene properties. Molecular weight of copolymers (Mw) can be in the range of 35,000 to 2,000,000, preferably 50,000 to 1,000,000, more preferably 70,000 to 500,000.
In a further aspect, this invention provides metallocene catalysts for propylene polymerization, having the structure
{ligand1-bridge-ligand2}MX2,
wherein
ligand1 and ligand2 are different and are selected from the group consisting of substituted and unsubstituted cyclopentadienyl (Cp), indenyl (ind), fluorenyl (flu), 4,5-dihydrocyclopentaphenanthryl (H2CPA), and cyclopentaphenanthryl (CPA) ring groups, wherein, when present, ring group substituents can be selected from the group consisting of
i) C1-C4 straight-chain or branched alkyl,
ii) C6-C20 aryl,
iii) C7-C20 alkylaryl,
iv) C4-C7 cycloalkyl,
v) (xe2x80x94CH2xe2x80x94)n, wherein n is 2, 3, 4, or 5, or (xe2x80x94CHxe2x95x90CHxe2x80x94)m wherein m is 1, 2, 3, or 4, connecting two positions (i.e., adjacent or non-adjacent ring carbon atoms) in the same ring structure, preferably the 4 and 5 positions in fluorenyl or indenyl,
vi) fused aromatic rings, and
vii) fused aromatic rings substituted by any one of groups i)-v);
bridge is a linking group joining ligand1 and ligand2 at C-1 of Cp or ind ligands or C-9 of flu and CPA ligands and is selected from the group consisting of
i)  greater than CR1R2,
ii)  greater than SiR1R2,
iii) xe2x80x94CR1R2xe2x80x94CR3R4xe2x80x94,
iv) xe2x80x94SiR1R2xe2x80x94SiR3R4xe2x80x94,
v) xe2x80x94CR1R2xe2x80x94SiR3R4xe2x80x94,
wherein R1, R2, R3, and R4 can be the same or different and are selected from the group consisting of H, C1-C20 straight-chain or branched alkyl groups, C6-C20 aryl groups, and C3-C8 cycloalkyl groups;
M is a metal atom selected from the group consisting of Zr, Hf, and Ti, preferably is zirconium or hafnium, and most preferably zirconium, and
X is selected from the group consisting of Cl, Br, I, C1-C20 straight-chain or branched alkyl groups, C6-C20 aryl groups, C7-C20 alkaryl groups, and C7-C20 aralkyl groups,
wherein the metallocene catalyst exhibits an asymmetry parameter of 1.03 to 1.69, preferably 1.03 to 1.45, more preferably 1.05 to 1.35,
with the proviso that the metallocene catalyst provides propylene polymers described above.
In a still further aspect, this invention also provides a method of making novel catalyst precursor compounds of the type
H{ligand1 xe2x80x94CH2CH2xe2x80x94 ligand2}H
wherein ligand1and ligand2 are as previously defined, the method comprising the steps:
1) deprotonating the hydroxyl group of a compound of the type ligand1-CH2CH2OH;
2) reacting the deprotonated compound with a perfluoroalkylsulfonyl fluoride of the type RfSO2F to obtain a stable perfluoroalkylsulfonate of the type
ligand1-CH2CH2xe2x80x94OSO2Rf;
wherein Rf means an alkyl group having 1-20 carbon atoms or an aryl group having 5 to 12 carbon atoms in which at least 75% of H atoms have been replaced by F atoms;
3) condensing the perfluoroalkylsulfonate with the conjugate base of a compound of the type
ligand2-H;
wherein ligand1 and ligand2 are as previous defined and Rf is independently selected from the group consisting of highly fluorinated or perfluorinated alkyl radicals containing from 1-20 carbon atoms.
In yet a further aspect, there is provided a novel class of metallocene catalysts that are particularly useful to prepare hybrid polymers of the present invention, i.e. those having intermediate stereoregularity between those that are highly syndiotatic (or highly isotactic) and those that are atactic, the novel catalysts of the present invention having the formula: 
wherein
strap can be a (xe2x80x94CH2xe2x80x94)n group or a (xe2x80x94CHxe2x95x90CHxe2x80x94)m group,
bridge is as previously defined,
Ma is Zr or Hf,
n is 1, 2, 3, or 4 and m is 1, 2, 3, or 4, and
X is as previously defined,
with the proviso that when Maxe2x95x90Zr, Xxe2x95x90Cl, and bridge=C2H4, then strap can (xe2x80x94CH2xe2x80x94)n or (xe2x80x94CHxe2x95x90CHxe2x80x94)m, wherein n=3, 4 or 5, and m =2, 3, or 4, preferably m is 2, 3, or 4 and can be designated mxe2x80x2.
In yet another aspect, this invention also provides a method of controlling the stereostructure of polypropylene by choosing the shape of a metallocene catalyst, as defined by its asymmetry parameter, from the class of catalysts having the structure
{ligand1-bridge-ligand2}MX2,
wherein
ligand1 and ligand2, bridge, M, and X are as previously defined, and wherein the metallocene catalyst exhibits an asymmetry parameter of 1.03 to 1.69, and the stereoregularity index of said polypropylene increases from about 1.30 to about 10.00 with an increase in the asymmetry parameter of the catalyst.
Preparation of the hybrid polypropylenes of the invention is accomplished by combining propylene with a metallocene catalyst, preferably in the absence of solvent for the monomer, in an inert atmosphere at a pressure in the range of 69 to 6890 Kpa at a temperature in the range of xe2x88x9220xc2x0 C. to about 1 20xc2x0 C. When a solvent is used, it can be a hydrocarbon, preferably aliphatic, cycloaliphatic, or aromatic, more preferably it is toluene or cyclohexane. Activation of the metallocene is accomplished by activators known in the art, preferably methylaluminoxane alone or in combination with trialkylaluminum compounds such as trimethylaluminum, triethylaluminum, or triisobutylaluminum.
As used herein:
xe2x80x9casymmetry parameter (A.P.)xe2x80x9d is defined as the ratio of the van der Waals surface area of the larger ligand to that of the smaller ligand, wherein the van der Waals surface area can be calculated by CAChe(trademark) Satellite molecular modeling program (version 3.8) (Oxford Molecular Ltd., Oxford, United Kingdom) on a Macintosh computer; this is a measure of the asymmetric character of a molecule;
xe2x80x9catacticxe2x80x9d means xe2x80x9cpolypropylene having no dominant tacticity and having 25% mm triads and 25% rr triads, as determined by NMR spectroscopy;
xe2x80x9cCp or cpxe2x80x9d means cyclopentadienyl;
xe2x80x9cCPAxe2x80x9d means cyclopentaphenanthryl;
xe2x80x9celastomericxe2x80x9d or xe2x80x9celasticxe2x80x9d means a material that at room temperature can be stretched repeatedly to at least twice its original length and, immediately upon release of the stress, returns with force to its approximate original length;
xe2x80x9centanglement molecular weight ({overscore (M)}e)xe2x80x9d means average molecular weight of chains between entanglements in a polymer;
xe2x80x9cfilmxe2x80x9d means a self-supporting layer;
xe2x80x9cfluxe2x80x9d means fluorenyl;
xe2x80x9cgroupxe2x80x9d or xe2x80x9cradicalxe2x80x9d or xe2x80x9ccompoundxe2x80x9d or xe2x80x9cligandxe2x80x9d or xe2x80x9cmonomerxe2x80x9d or xe2x80x9cpolymerxe2x80x9d means a chemical species that allows for substitution or which may be substituted by conventional substituents which do not interfere with the desired product; e.g., substituents can be alkyl, aryl, phenyl, etc.; and
xe2x80x9cH2CPAxe2x80x9d means 4,5-dihydrocyclopentaphenanthryl;
xe2x80x9chelical lengthxe2x80x9d means the length of a segment in a coiled configuration in contrast to the fully extended polymer segment;
xe2x80x9cheterotacticxe2x80x9d sequences are those having both r or m diads;
xe2x80x9chighly fluorinatedxe2x80x9d (Rf) means having at least 75% of H atoms in an alkyl cycloalkyl, or aryl group substituted by fluorine atoms;
xe2x80x9chighly isotacticxe2x80x9d means polypropylene having an mm content of at least 80%, as determined by NMR spectroscopy;
xe2x80x9chighly syndiotacticxe2x80x9d means polypropylene having an rr content of at least 80%, as determined by NMR spectroscopy;
xe2x80x9chomotacticxe2x80x9d sequences are those having diads which are all r or all m. xe2x80x9cindxe2x80x9d means indenyl;
xe2x80x9cisotacticxe2x80x9d or xe2x80x9cisotactic-richxe2x80x9d means polypropylene having tacticity shown in FIG. 2 (second entry) and having an mm content of greater than the rr triad content, as determined by NMR spectroscopy;
xe2x80x9cMexe2x80x9d means methyl;
xe2x80x9cmetallocenexe2x80x9d means a metal-organic compound characterized by xcfx80-bonds between a transition metal and a cyclopentadienyl, indenyl, or fluorenyl ligand, or such substituted ligands;
xe2x80x9cmm triadsxe2x80x9d refers to an isotactic structure having three propylene units placed in a polypropylene backbone such that all three methyl side groups lie on the same side of the plane containing the polymer backbone, as determined by NMR spectroscopy;
xe2x80x9cmr triadsxe2x80x9d refers to a stereoregular polypropylene structure having three successive propylene units placed in a polypropylene backbone such that, relative to the first propylene unit, the methyl group of the second propylene lies on the same side of the plane containing the polymer backbone and the methyl group of the third propylene lies on the opposite side of the plane containing the polymer backbone, as determined by NMR spectroscopy;
xe2x80x9cnanocystallinexe2x80x9d means having crystallites in the nanometer size range (largest length), preferably 2 to 200 nm;
xe2x80x9coptical clarityxe2x80x9d means transmittance of greater than 80% of light of wavelengths of 400-750 nm;
xe2x80x9cpeak molecular weight,xe2x80x9d Mp, means the maximum molecular weight in a curve relating molecular weight and the abundance of a species of a given molecular weight determined by gel phase (or size exclusion) chromatography;
xe2x80x9cPhxe2x80x9d means phenyl;
xe2x80x9crr triadsxe2x80x9d refers to a syndiotactic structure having three propylene units placed in a polypropylene backbone such that each successive methyl side group is alternately above and below the plane containing the polymer backbone, as determined by NMR spectroscopy;
xe2x80x9csolublexe2x80x9d means dissolves to an extent of greater than 98 weight percent at a temperature up to the boiling point of the stated solvent;
xe2x80x9cstereoregularity index (S.I.)xe2x80x9d of a propylene polymer is defined as the ratio of the percentage of mm triads to rr triads, wherein the ratio represents the larger of mm or rr over the smaller of mm or rr (i. e., the ratio is positive and greater than 1); and
xe2x80x9csyndiotacticxe2x80x9d or xe2x80x9csyndiotactic-richxe2x80x9d means polypropylene having tacticity shown in FIG. 2 (third entry) and having an rr content of greater than the mm triad content, as determined by NMR spectroscopy.
Metallocene catalysts that produce highly isotactic or highly syndiotactic polymers exhibit a number of chemical and/or structural similarities:
1) These catalysts very often contain a MX2 unit (M=Ti, Zr, or Hf; X=Cl, Br or CH3, most commonly Cl) bonded to two variously substituted cyclopentadienyl-type ligands such as cyclopentadienyl itself, 1-indenyl or 9-fluorenyl.
2) The two cyclopentadienyl-type ligands may be connected by a bridging group joined at the C-1 position in the indenyl moiety or the C-9 position in the fluorenyl moiety. The groups xe2x80x94CH2xe2x80x94CH2xe2x80x94 and xe2x80x94SiMe2xe2x80x94, wherein Me=methyl, are common examples.
3) During the course of activation by an organoaluminum compound such as methylaluminoxane or mixtures of methylaluminoxane and other co-catalysts, Cl is replaced by one or two alkyl groups derived from the organoaluminum compound so that equivalent results for compounds containing a MCl2 or M(Me2) moiety are often seen.
4) Once a basic catalyst structural type that provides isotactic or syndiotactic polymer has been recognized, stereoregularity can be increased by further adjustments of the structure through introduction of organic groups on the cyclopentadienyl-type ligands.
However, prior to the present invention it was not possible to predict the physical properties of polypropylene of intermediate stereoregularity based on the shape of the metallocene catalyst.
Metallocene catalysts of the invention produce novel propylene polymers, herein sometimes referred to as hybrid polymers, whose stereoregularity lies between highly syndiotactic (or highly isotactic) and atactic. Polymers thus obtained exhibit novel and useful properties, for example resistance to creep at high temperatures or extreme extensibility, which properties have not been previously observed in polypropylene.